If nothing is done to reduce emissions, current climate models predict a global warming of about
1.4 – 5.8°C between 1990 and 2100. These projections are based on a wide range of
assumptions about the main forces driving future emissions (such as population growth and
technological change) but do not assume any climate change policies for reducing emissions. Even a
1.4oC rise would be larger than any century-time-scale trend for the past 10,000 years.
These projections takes into account the effects of aerosols and the delaying effect of the oceans.
Oceanic inertia means that the earth's surface and lower atmosphere would continue to warm for
hundreds of years even if greenhouse gas concentrations stopped rising in 2100.

The average sea level is predicted to rise by 9 to 88 cm by 2100. This would be caused mainly
by the thermal expansion of the upper layers of the ocean as they warm, with some contribution from
melting glaciers. The uncertainty range is large, and changing ocean currents, local land movement
and other factors could cause local and regional sea levels to rise much more or much less than the
global average. Slightly faster melting of the Greenland and Antarctica ice sheets is likely to be
balanced counteracted by increased snowfall in both regions. As the warming penetrates deeper into
the oceans and ice continues to melt, the sea level will continue rising well long after surface
temperatures have leveled off.

Regional and seasonal warming predictions are much more uncertain. Although most areas are
expected to warm, some will warm much more than others. The largest warming is predicted for cold
northern regions in winter. The reason is that snow and ice reflect sunlight, so less snow means more
heat is absorbed from the sun, which enhances any warming: a strong positive feedback effect. By the
year 2100, winter temperatures in northern Canada, Greenland and northern Asia are predicted to rise
by 40% more than the global average.

Inland regions are projected to warm faster than oceans and coastal zones. The reason is
simply the ocean delay, which prevents the sea surface from warming as fast as the land. The size of
this delay depends on how deep any warming penetrates into the oceans. Over most of the oceans, the
uppermost few hundred metres do not mix with the water beneath them. These upper layers will warm
within just a few years, while the deep ocean stays cold. Water mixes down into the ocean depths in
only a few very cold regions, such as the Atlantic south of Greenland and the Southern Ocean near
Antarctica. In these regions, warming will be delayed because much more water needs to be warmed up
to get the same temperature change at the surface.

Global precipitation is predicted to increase, but at the local level trends are much less
certain. By the second half of the 21st century, it is likely that wintertime
precipitation in the northern mid- to high latitudes and in Antarctica will rise. For the tropics,
models suggest that some land areas will see more precipitation, and others less. Australia, Central
American and southern Africa show consistent decreases in winter rainfall.

More rain and snow will mean wetter soil conditions in high-latitude winters, but higher
temperatures may mean drier soils in summer. Local changes in soil moisture are clearly important
for agriculture, but models still find it difficult to simulate them. Even the sign of the global
change in summertime soil moisture - whether there will be an increase or a decrease - is
uncertain.

The frequency and intensity of extreme weather events such as storms and hurricanes may are likely
to change. With increasing global temperatures the world is likely to experience more hot days
and heat waves and fewer frost days and cold spells. Climate models also consistently show extreme
precipitation events becoming more frequent over many areas and the risk of drought becoming greater
over continental areas in summer. There is also some evidence to show that hurricanes could be more
intense (with stronger winds and more rainfall) in some areas. There is little agreement amongst
models concerning changes in mid-latitude storms. There are also other phenomena, such as
thunderstorms and tornadoes, where knowledge is currently inadequate for making projections. However,
models still cannot predict how. The models used to simulate climate change cannot themselves
simulate these extreme weather events, so the evidence is indirect. There is some concern that
patterns of extreme weather may change because the models predict changes in ocean surface
temperatures and other factors that are known to affect storm and hurricane development. However, it
will be many years before scientists can predict whether individual regions will become more or less
stormy.

Rapid and unexpected climate transitions cannot be ruled out. The most dramatic such change,
the collapse of the West Antarctic ice sheet, which would lead to a catastrophic rise in sea level,
is now considered unlikely during the 21st century. There is evidence that changes in
ocean circulation having a significant impact on regional climate (such as a weakening of the Gulf
Stream that warms Europe) can take place in only a few decades, but it is unknown whether or not
greenhouse warming could trigger any such change. Climate models that do show a weakening in the Gulf
Stream still project warming over Europe.